The epidermal growth factor precursor (proEGF) is a large membrane protein containing nine EGF units in the extracellular domain. The EGF unit closest to the membrane corresponds to mature EGF. Proteolytic cleavage at the C-terminus of the extracellular domain generates a soluble fragment. Sveral other membrane-anchored growth factors, members of the EGF gene family, undergo similar proteolytic cleavage and subsequent release. The proteases responsible for this growth factor maturation have not yet been identified. Some information is available regarding the amino acid sequence bounding the putative cleavage site. A consensus oligonucleotide primer encoding this region and a primer encoding part of the EGF unit are being used for PCR screening of several lambda gt11 cDNA libraries as a first step to determine the ubiquity of this cleavage site.